In general, synthetic resins have various functions depending on their use and the conditions to which the resins are subjected. Conventional synthetic resins are modified by adding functional monomers or polymers to the synthetic resins to impart novel functions. Thermoplastic resins are generally characterized by many advantageous properties which include optical clarity, high ductility, high heat deflection temperature and as dimensional stability. As a result of such properties they are often employed in many industrial applications. Aromatic polycarbonates are described in U.S. Pat. No. 4,172,103. Aromatic polycarbonates are prepared by reacting a dihydric phenol (example—2,2-bis(4-hydroxyphenyl) propane) with a carbonate precursor such as phosgene, in the presence of an acid binding agent. In general, aromatic polycarbonates exhibit good resistance to attack by mineral acids, are readily moldable, and are physiologically inert.
Polydioranosiloxane/polycarbonate block copolymers are described in U.S. Pat. Nos. 3,189,662, 3,821,325, 3,832,419 and 3,679,774. Polydioranosiloxane/polycarbonate block copolymers are used as coatings, insulating materials, coverings, binding materials and adhesives.
U.S. Pat. No. 3,419,634 discloses polydiorganosiloxane/polycarbonate block copolymers with unsaturated end groups, which can be cured and which can contain fillers. These block copolymers can be used inter alia as coverings, car windows, binders for safety glass and seals for glass.
While thermoplastic resins possess many advantageous properties, they are susceptible to photodegradation by ultraviolet light. The photodegradation results in unfavorable characteristics including yellowing and erosion of the polymer surface. There are different ways to prepare thermoplastic resins, such as polycarbonate with improved UV resistance properties. One of the methods employs treating a polycarbonate surface with a coating material, containing ultraviolet absorbing agents. This approach suffers from poor compatibility of the UV absorbing agents with the coating materials which limits the amount of UV absorbing agent that can be employed in coating. Extra steps involved in coating polycarbonates increase the cost of manufacturability. Another method involves the addition of UV absorbing materials as an additive during the polycarbonate processing step. Even though such a process avoids the additional coating step, UV stabilizers can be added only in very small quantities (in ppm levels) without affecting the optical clarity of polycarbonate article. Thermal degradation of UV stabilizers is also a limitation at very high processing temperature of polycarbonate (275° C. to 300° C.).
WO2008/00084 A1 discloses that UV absorbers that can be used in mixtures of high-molecular aromatic polycarbonate with up to 50 percent by weight of a block copolymer consisting of alternating segments of aromatic polycarbonate and polysiloxane in a weight ratio of 25:75 to 75:25. The mixtures are distinguished by a high ductility, solvent resistance and improved fire behavior.
However, the known additive approach in the prior art for improving the UV resistance of polycarbonate and/or polycarbonate-polysiloxane copolymers or polymer blends has the disadvantage of additive leachability and additional processing steps. Accordingly, there is a need for polysiloxane compositions that can provide improved UV resistance with a reduced number of processing steps. The present invention provides answer to that need.